Internal Geneva Mechanism

AIM : To create a 3-D model of internal geneva mechanism and perform motion analysis on Solidworks.

OBJECTIVE :

1. To create 3-D model of driven and driving wheel according to problem statement.
2. To perform it's motion analysis at 10 rpm and 20 rpm.
3. To plot graphs of contact forces(between driving and the driven wheel), with and without precise contact, angular displacement of driven wheel, angular velocity of driver's wheel as a function of time for both speeds mentioned above.

THEORY :

Geneva mechanism, also called Geneva Stop, one of the most commonly used devices for producing intermittent rotary motion, characterized by alternate periods of motion and rest with no reversal in direction. It is also used for indexing (i.e., rotating a shaft through a prescribed angle). Geneva mechanism is commonly used indexing mechanism where an intermittent motion is required. The Inverse Geneva mechanism, which is a variation of the Geneva mechanism, is used where the wheel has to rotate in the same Direction as crank. It requires less radial space and the locking device can be a circular segment attached to the crank that locks by wiping against a built up rim on the periphery of the wheel. The design and fabricating of a conventional Geneva mechanism is generally simple and inexpensive because there is no specially curved profile on any of the components except straight lines and circular arcs. However, due to the discontinuity of the acceleration at the beginning and ending positions, the shortcoming of using conventional Geneva mechanism is the large impact when the driving crank engages and disengages with the wheel slot.

Geneva Mechanism

CLASSIFICATION OF GENEVA MECHANISM :

1. External geneva mechanism,
2. Internal geneva mechanism,
3. Spherical geneva mechanism

External geneva mechanism Internal geneva mechanism Spherical geneva mechansim

ADVANTAGES OF GENEVA MECHANISM :

1. Geneva mechanism may be the simplest and least Expensive of all intermittent motion mechanisms.
2. They come in a wide variety of sizes, ranging from those used in instruments, to those used in machine tools to index spindle carriers weighing several tons.
3. They have good motion curves characteristics compared to ratchets, but exhibit more “jerk” or instantaneous change in acceleration, than better cam systems.
4. Geneva maintains good control of its load at all Times, since it is provided with locking ring surfaces.

DISADVANTAGES OF GENEVA MECHANISM :

1. The Geneva is not a versatile mechanism.
2. The ratio of dwell period to motion is also established Once the no of dwells per revolution has been selected.
3. All Geneva acceleration curves start and end With finite acceleration & deceleration.
4. This means they produce jerk.

APPLICATIONS & USES : Stepper, Mechanical watches, Plotters, CNC machine, Iron ring clock

PROCEDURE :

• 3D MODELLING AND ASSEMBLY -

1. At first, 3D model of driven and driving wheel are constructed according to problem statement as shown here,
2. After making these model, it has to be assembled accordingly to achieve it's desired motion. For this, the 3D models designed are imported into assembly window of solidworks. The first component inserted will be fixed. To unfix this, the first input part will be made to float by right clicking it.
3. For driving wheel, first it has to be made coincident with front plane of the assembly( as this part is itself made in sam eplane), then temproary axis is made on for better viewings. Now the central axis of driving wheel is made coincident with right plane and top plane of the assembly. This ensures that it will not translate in any other planes and will rotate about its axis in front plane.
4. Now for driven wheel, same steps is repeated and one extra mating is done between driven wheel and driving wheel based on their geometry. This ensures that driven wheel will also rotate about its cental axis and will not translate in any other planes. Then the positions of both driving and driven wheels are set manually so that their is some clearance between them and does not overlap each other.

View of the assembly from the front plane

View of the assembly from the right plane

• MOTION STUDY AND ANALYSIS :

1. After assembly of components, for motion study the add-ins in Solidworks is clicked and Solidworks motion is selected. Then a motor/torque is provided to the driving wheel as required. Contact between solid bodies are provided by choosing required solid bodies and their material is selected as Steel(dry).
2. Then solve and calculate button is pressed, it will run and solve the analysis. After this the plots can be made from the results obtained by choosing desired parameters.

• ANALYSIS AT 10 rpm :

• For contact force between driven and driving wheel at 10 rpm(without precise contact), plots were made by using results and imported to excel spreadsheet. The following is the result obtained.

• It depicts that heavy jerks are produced when the shaft of driving wheel is entering the slots of driven wheel.The contact force is continuously changing in a given time interval

• For angular displacement of driven wheel at 10 rpm, plots were made by using results and imported to excel spreadsheet. The following is the result obtained.

• Angular displacement of driven wheel experiences a gradual decrement and then slight increment over a cycle.

• For angular velocity of driver wheel at 10 rpm, plots were made by using results and imported to excel spreadsheet. The following is the result obtained.

• For contact force between driven and driving wheel at 10 rpm(with precise contact), plots were made by using results and imported to excel spreadsheet. The following is the result obtained.

• The contact force is continuously changing in a given time interval and we can calculate each and every point of a jerk as shown in the plot. Precise contact gives a more accurate result.

• Image showing selection of precise contact option -

• ANALYSIS AT 20 rpm :

• For contact force between driven and driving wheel at 20 rpm(without precise contact), plots were made by using results and imported to excel spreadsheet. The following is the result obtained.

• For angular displaceent of driven wheel at 20 rpm, plots were made by using results and imported to excel spreadsheet. The following is the result obtained.

• For angular velocity of driver wheel at 20 rpm, plots were made by using results and imported to excel spreadsheet. The following is the result obtained.

• For contact force between driven and driving wheel at 20 rpm(with precise contact), plots were made by using results and imported to excel spreadsheet. The following is the result obtained.

RESULTS :

1. The 3D model of the geometry can be found in the google drive. Google Drive Link
2. The youtube link of the animation of the assembly at 10 rpm - Link 1
3. The youtube link of the animation of the assembly at 20 rpm - Link 2

• CONCLUSIONS MADE FROM ANALYSIS :

1. When the driver wheel's shaft enter in the slot of the driven wheel it faces a jerk as the entry is not smooth therefore,we can see also from the plot that at the entry to the slot gives rise to a suddent increment in the contact force which decreases as the driver enters into the slot.

2. The angular displacement plot shows that the displacement occurs between -180 to 180 degree and it repeats the pattern in each revolution starting from 0.

3. The Precise Contact gives a more accurate result as we can see that the curves in the plots with precise contact becomes less fluctuating and smooth.

4. The angular velocity of the driver wheel also increases if we increase its rpm. There is a jerk felt by the driver wheel when it enters the slot.

5. As the rpm of the driving wheel increases the jerk produced during the motion study also decreases simultaneously.

• SOURCES OF ERROR : Heavy jerks may be obtained during motion study, it can be due to misalignment between driven and driver wheel or wrong placemnets of them. If these are avoided the plots obtained will be more accurate.

REFERENCES :

1. https://learnmech.com/introduction-to-geneva-mechanis/
2. gif for internal geneva mechansim
3. image for external geneva mechanism
4. image for spherical geneva mechanism